There was an error in this gadget

## Thursday, September 25, 2008

### Angular diameter

After we talk about parallax, now we will discuss about angular diameter.

I. Definition

The angle that the actual diameter of an object makes in the sky; also known as angular size or apparent diameter. The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its center that is perpendicular to the viewing direction. Because of foreshortening, it may be quite different from the actual physical diameter for an object that is seen under an angle. For a disk-shaped object at a large distance, the visual and actual diameters are the same.The Moon, with an actual diameter of 3,476 kilometers, has an angular diameter of 29' 21" to 33' 30", depending on its distance from Earth. If both angular diameter and distance are known, linear diameter can be easily calculated.

The Sun and the Moon have angular diameters of about half a degree, as would a 10-centimeter (4-inch) diameter orange at a distance of 11.6 meters (38 feet). People with keen eyesight can distinguish objects that are about an arc minute in diameter, equivalent to distinguishing between two objects the size of a penny at a distance of 70 meters (226 feet). Modern telescopes allow astronomers to routinely distinguish objects one arc second in diameter, and less. The Hubble Space Telescope, for example, can distinguish objects as small as 0.1 arc seconds. For comparison, 1 arc second is the apparent size of a penny seen at a distance of 4 kilometers (2.5 miles).

The angular diameter is proportional to the actual diameter divided by its distance. If any two of these quantities are known, the third can be determined.

For example if an object is observed to have an apparent diameter of 1 arc second and is known to be at a distance of 5,000 light years, it can be determined that the actual diameter is 0.02 light years.

II. Formulas

The angular diameter of an object can be calculated using the formula:

in which δ is the angular diameter, and d and D are the visual diameter of and the distance to the object, expressed in the same units. When D is much larger than d, δ may be approximated by the formula δ = d / D, in which the result is in radians.

For a spherical object whose actual diameter equals dact, the angular diameter can be found with the formula:

for practical use, the distinction between d and dact only makes a difference for spherical objects that are relatively close.

III. Estimating Angular Diameter

This illustration shows how you can use your hand to make rough estimates of angular sizes. At arm's length, your little finger is about 1 degree across, your fist is about 10 degrees across, etc. Credit: NASA/CXC/M.Weiss

IV. Use in Astronomy

In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes.

The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2" (two arcseconds).

The angular diameter of the Sun, from a distance of one light-year, is 0.03", and that of the Earth 0.0003". The angular diameter 0.03" of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth.[1]

This table shows the angular sizes of noteworthy celestial bodies as seen from the Earth:

 Sun 31.6' – 32.7' Moon 29.3′ – 34.1' Venus 10″ – 66″ Jupiter 30″ – 49″ Saturn 15″ – 20″ Mars 4″ – 25″ Mercury 5″ – 13″ Uranus 3″ – 4″ Neptune 2″ Ceres 0.8″ Pluto 0.1″

* Betelgeuse: 0.049″ – 0.060″
* Alpha Centauri A: ca. 0.007″
* Sirius: ca. 0.007″

This meaning the angular diameter of the Sun is ca. 250,000 that of Sirius (it has twice the diameter and the distance is 500,000 times as much; the Sun is 10,000,000,000 times as bright, corresponding to an angular diameter ratio of 100,000, so Sirius is roughly 6 times as bright per unit solid angle).

The angular diameter of the Sun is also ca. 250,000 that of Alpha Centauri A (it has the same diameter and the distance is 250,000 times as much; the Sun is 40,000,000,000 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle).

The angular diameter of the Sun is about the same as that of the Moon (the diameter is 400 times as large and the distance also; the Sun is 200,000-500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450-700, so a celestial body with a diameter of 2.5-4" and the same brightness per unit solid angle would have the same brightness as the full Moon).

Even though Pluto is physically larger than Ceres, when viewed from Earth, e.g. through the Hubble Space Telescope, Ceres has a much larger apparent size.

While angular sizes measured in degrees are useful for larger patches of sky (in the constellation of Orion, for example, the three stars of the belt cover about 3 degrees of angular size), we need much finer units when talking about the angular size of galaxies, nebulae or other objects of the night sky.

Degrees, therefore, are subdivided as follows:

* 360 degrees (º) in a full circle
* 60 arc-minutes (′) in one degree
* 60 arc-seconds (′′) in one arc-minute

To put this in perspective, the full moon viewed from earth is about ½ degree, or 30 arc minutes (or 1800 arc-seconds). The moon's motion across the sky can be measured in angular size: approximately 15 degrees every hour, or 15 arc-seconds per second. A one-mile-long line painted on the face of the moon would appear to us to be about one arc-second in length.

Source : Wikipedia and encyclopedia of science.

Cited from : All About Astronomy

## Saturday, September 20, 2008

### Parallax : Distance Measurement

Before we learn further about astronomy, there are some basic knowledges that we must know and understand.

First, we will talk about measuring distance in astronomy.

Astronomical object lies in a very great distance from us. So far than our sense can perceive. That's why our sense can't have a 3-D visualization of the universe. Our sense can't differ closer to farther objects. So, we need some trick to know how far an object from us. One of the simplest method used by astronomers to measure distance of some closest star is using the parallax effect.

Parallax is an optical effect seen when the observer seeing an object from two different positions. The object will be seen shifted relative to the farther background objects.

The parallax effect is one of those things you see everyday and think nothing of until it's given some mysterious scientific-sounding name. There's really no magic here. Consider the following simple situation.

You're riding in a car on a highway out west. It's a beautiful sunny day, and you can see for miles in every direction. Off to your left, in the distance, you see a snow-capped mountain. In front of that mountain, and much closer to the car, you see a lone ponderosa pine standing in a field next to the highway. I've diagramed this idyllic scene in the figure below:

As you drive by the field, you notice an interesting sight. When you're in the position on the left side of the figure, the tree appears to be to the right of the mountain. You can see this in the figure by the fact that the line of sight to the tree (indicated by the green line) is rightward of the line of sight to the mountain (indicated by the blue line). A picture of what you see out the window of your car is shown below the car.

The interesting part is that as your drive on, you notice that the tree and mountain have switched positions; that is, by the time you reach the right hand position in the above figure, the tree appears to be to the left of the mountain. You can see this in the figure by noting that the line of sight to the tree (green line) is leftward of the line of sight to the mountain (blue line). A picture of what you see out the window of your car now is shown below the car.

What's going on here? It's pretty clear that the tree and mountain haven't moved at all, yet the tree appears to have jumped from one side of the mountain to the other. By now, you're probably saying "Well, DUH, the tree is just closer to me than the mountain. What's so remarkable about that?" I would answer, "There's nothing at all remarkable about it. It's just the effect of parallax." In fact, if you understand the above discussion, you already understand the parallax effect.

Now let's talk about measuring the distance to the tree using this information. From the above information, you can see that it would be pretty easy to measure the angle between the direction to the tree and the direction to the mountain in both instances. Let's call those angles A and B, respectively. Now, if the mountain is sufficiently distant so that the direction to the mountain from both viewpoints is the same, then the two blue lines in the figure below are parallel.

This helps a lot, because we can then show that the angle made by the two green lines (i.e., the difference in the direction to the pine tree from the two viewpoints) is equal to the sum of A and B. To see this, construct a line through the pine tree parallel to the two blue lines in the figure (this line is shown as a dotted line above). Then all of the blue lines are parallel, and each of the green lines crosses a pair of parallel lines. Reach deep back into your high school geometry (or equivalently, just stare at the above figure for a minute), and you'll remember or realize that the angles at the pine tree labeled A and B have the same values as the angles A and B measured at the two car positions. Thus, the angle between the two green lines is the sum of A and B, which are angles we can measure from the comfort of our car.

Now, if we know the distance D we've traveled, then we have an Observer's Triangle and we can solve for the distance to the tree using the Observer's Triangle relation

alpha/57.3 = D/R where alpha is the angle at the tree (A + B), D is the distance we've traveled between views, and R is the distance from the road to the tree. (source : Astronomy 101 Specials: Measuring Distance via the Parallax Effect).

We will use the same method to measure the star's distance. This method is called trigonometric parallax because we only use simple triangulation to find the distance. The only problem is star's distance is so huge so the parallax effect will be so small (less than 1 arc second; 1 arc second = 1/3600 of a degree). So, that's why this method can only measure accurately for several nearby stars. Farther star will need different, more complex, indirect method to derive its distance.

As explained before, the stars are so far away that observing a star from opposite sides of the Earth would produce a parallax angle much, much too small to detect (That's why ancient people can't detect this shifting to prove heliocentric view) . As a consequence, we must use the largest possible baseline. The largest one that can be easily used is the orbit of the Earth. In this case the baseline is the mean distance between the Earth and the Sun---an astronomical unit (AU) or 149.6 million kilometers! A picture of a nearby star is taken against the background of stars from opposite sides of the Earth's orbit (six months apart). The parallax angle p is one-half of the total angular shift.

However, even with this large baseline, the distances to the stars in units of astronomical units are huge, so a more convenient unit of distance called a parsec is used (abbreviated with "pc''). A parsec is the distance of a star that has a parallax of one arc second using a baseline of 1 astronomical unit. Therefore, one parsec = 206,265 astronomical units. The nearest star is about 1.3 parsecs from the solar system. In order to convert parsecs into standard units like kilometers or meters, you must know the numerical value for the astronomical unit---it sets the scale for the rest of the universe. Its value was not know accurately until the early 20th century. In terms of light years, one parsec = 3.26 light years.

Using a parsec for the distance unit and an arc second for the angle, our simple angle formula above becomes extremely simple for measurements from Earth:

p = 1/d

Parallax angles as small as 1/50 arc second can be measured from the surface of the Earth. This means distances from the ground can be determined for stars that are up to 50 parsecs away. If a star is further away than that, its parallax angle p is too small to measure and you have to use more indirect methods to determine its distance. Stars are about a parsec apart from each other on average, so the method of trigonometric parallax works for just a few thousand nearby stars. The Hipparcos mission greatly extended the database of trigonometric parallax distances by getting above the blurring effect of the atmosphere. It measured the parallaxes of 118,000 stars to an astonishing precision of 1/1000 arc second (about 20 times better than from the ground)! It measured the parallaxes of 1 million other stars to a precision of about 1/20 arc seconds. Selecting the Hipparcos link will take you to the Hipparcos homepage and the catalogs.

The actual stellar parallax triangles are much longer and skinnier than the ones typically shown in astronomy textbooks. They are so long and skinny that you do not need to worry about which distance you actually determine: the distance between the Sun and the star or the distance between the Earth and the star. Taking a look at the skinny star parallax triangle above and realizing that the triangle should be over 4,500 times longer (!), you can see that it does not make any significant difference which distance you want to talk about. If Pluto's entire orbit was fit within a quarter (2.4 centimeters across), the nearest star would be 80 meters away! But if you are stubborn, consider these figures for the planet-Sun-star star parallax triangle setup above (where the planet-star side is the hypotenuse of the triangle):

the Sun -- nearest star distance = 267,068.230220 AU = 1.2948 pc

the Earth--nearest star distance = 267,068.230222 AU = 1.2948 pc

Pluto--nearest star distance = 267,068.233146 AU = 1.2948 pc!

If you are super-picky, then yes, there is a slight difference but no one would complain if you ignored the difference. For the more general case of parallaxes observed from any planet, the distance to the star in parsecs d = ab/p, where p is the parallax in arc seconds, and ab is the distance between the planet and the Sun in AU.

Formula (1) relates the planet-Sun baseline distance to the size of parallax measured. Formula (2) shows how the star-Sun distance d depends on the planet-Sun baseline and the parallax. In the case of Earth observations, the planet-Sun distance ab = 1 A.U. so d = 1/p. From Earth you simply flip the parallax angle over to get the distance! (Parallax of 1/2 arc seconds means a distance of 2 parsecs, parallax of 1/10 arc seconds means a distance of 10 parsecs, etc.)

A nice visualization of the parallax effect is the Distances to Nearby Stars and Their Motions lab (link will appear in a new window) created for the University of Washington's introductory astronomy course. With this java-based lab, you can adjust the inclination of the star to the planet orbit, change the distance to the star, change the size of the planet orbit, and even add in the effect of proper motion. (source : www.astronomynotes.com)

Units in Distance

1. Astronomical Unit (A.U). It is defined as the mean distance of the Sun from the Earth. Its value is about 149,6 million km. This unit is conveniently used to express distance to the object in solar system because we can directly compared the distance to Earth-Sun distance.

2. One light year is defined as the distance that light has traveled in light years. Light has velocity about 300.000 km/s. So, one light year equals to 9,46 x 10^12 km. This unit is mostly used to express the distance of extragalactic object. Remember that light's speed is finite so distant objects are seen as they are in the past. For example the Sun. The Sun that we see at this moment is the Sun as it was 8 minutes ago. Light needs about 8 minutes to travel the Earth-Sun distance. So, looking farther objects mean we're looking even further to the past. That's why light years is more commonly used to express distant object's distance. When we say that a cluster's distance is 8 billion light years, it means that the cluster that we seen right now is the way it looks 8 billion years ago !

3. Parsec (Parallax second). Star that have parallax 1 arc second have distance about 3,26 light years or 206.265 A.U (astronomical unit). Astronomer use this distance as a unit to express distance of the star. It is called a parsec. This unit is favorable to express star's distance because it is closely related to star's parallax (p). (remember that parallax = 1/distance, while the observer is on Earth, parallax is expressed in arc second and distance is expressed in parsec).

So, for reviewing our understanding about the parallax, try to answer these questions:

1. If a star has parallax 0",711, determine its distance (in light years) from us!

2. Assume we can measure parallax from Mars (with the same technology that we used here on Earth). Assume that we can measure accurately using parallax method until 200 parsec from the Earth (distance limit). Determine the distance limit if we conduct the measurement of star's distance using parallax method. Given that the distance of Mars from the Sun is about 1,52 AU.

3. You observe an asteroid approaching the Earth. You have two observatories 3200 km apart, so you can measure the parallax shift of the incoming asteroid. You observe the parallax shift to be 0,022 degrees.Determine : (a) the parallax expressed in radians (b) the asteroid's distance from Earth.

4. If you measure the parallax of a star to be 0,1 arc seconds on Earth, how big would the parallax of the same star for an observer on Mars?

5. If you measure the parallax of a star to be 0,5 arc seconds on Earth and an observer in a space station in the orbit around the Sun measures a parallax for the same star to be 1 arc seconds, how far is the space station from the Sun ?

You can share your solution of the above questions in the comment column.

## Wednesday, September 17, 2008

### Sideralis

Program berbasis Java© ini bisa kompatibel dengan semua handphone yang mendukung CLDC1.1 MIDP2.0. Program ini termasuk berukuran kecil (hanya sekitar 128 kb) tetapi dapat menampilkan simulasi langit real-time di layar handphone. Jadi, jika kebetulan Anda sedang mengamati langit, program ini dapat selalu memberi informasi kepada Anda setiap saat karena dapat diakses lewat HP Anda.

Beberapa features-nya :
• A lot of different objects:
• More than 800 stars.
• 5 planets: Mars, Venus, Mercury, Jupiter and Saturn.
• Sun and Moon.
• 2 different views :
• Horizontal view.
• Zenith view.
• Information on all objects:
• Name.
• Magnitude
• Distance
• Azimuth
• Height
• Current time or any time can be choosen.
• Selection of location by either entering latitude and longitude, by selecting a city among than 50 cities or by selection on a globe.
• Small dictionnary available giving some interesting informations about universe.
Program tersebut dapat didownload dari link yang ada di bawah ini. Semoga dapat bermanfaat. Sekarang semua bisa punya planetarium mini dalam handphone mereka. Semoga software sejenis ini dapat lebih banyak lagi agar astronomi dapat semakin memasyarakat.

Sideralis

## Monday, September 15, 2008

### "Laser Comb" To Measure the Accelarating Universe

Back in April, UT published an article about using a device called a 'laser comb' to search for Earth-like planets. But astronomers also hope to use the device to search for dark energy in an ambitious project that would measure the velocities of distant galaxies and quasars over a 20-year period. This would let astronomers test Einstein's theory of general relativity and the nature of the mysterious dark energy. The device uses femto-second (one millionth of one billionth of a second) pulses of laser light coupled with an atomic clock to provide a precise standard for measuring wavelengths of light. Also known as an “astro-comb,” these devices should give astronomers the ability to use the Doppler shift method with incredible precision to measure spectral lines of starlight up to 60 times greater than any current high-tech method. Astronomers have been testing the device, and hope to use one in conjunction with the new Extremely Large Telescope which is being designed by ESO, the European Southern Observatory.

Astronomers use instruments called spectrographs to spread the light from celestial objects into its component colors, or frequencies, in the same way water droplets create a rainbow from sunlight. They can then measure the velocities of stars, galaxies and quasars, search for planets around other stars, or study the expansion of the Universe. A spectrograph must be accurately calibrated so that the frequencies of light can be correctly measured. This is similar to how we need accurate rulers to measure lengths correctly. In the present case, a laser provides a sort of ruler, for measuring colors rather than distances, with an extremely accurate and fine grid.

New, extremely precise spectrographs will be needed in experiments planned for the future Extremely Large Telescope.

"We'll need something beyond what current technology can offer, and that's where the laser frequency comb comes in. It is worth recalling that the kind of precision required, 1 cm/s, corresponds, on the focal plane of a typical high-resolution spectrograph, to a shift of a few tenths of a nanometre, that is, the size of some molecules," explains PhD student and team member Constanza Araujo-Hauck from ESO.

The new calibration technique comes from the combination of astronomy and quantum optics, in a collaboration between researchers at ESO and the Max Planck Institute for Quantum Optics. It uses ultra-short pulses of laser light to create a 'frequency comb' - light at many frequencies separated by a constant interval - to create just the kind of precise 'ruler' needed to calibrate a spectrograph.

The device has been tested on a solar telescope, a new version of the system is now being built for the HARPS planet-finder instrument on ESO's 3.6-metre telescope at La Silla in Chile, before being considered for future generations of instruments.

Sumber : ESO

## Saturday, September 13, 2008

### Universe

Description :
# Paperback: 800 pages
# Publisher: W.H.Freeman & Co Ltd; 8th Revised edition (30 Jul 2007)
# Language English
# ISBN-10: 0716795647
# ISBN-13: 978-0716795643
# Product Dimensions: 27.2 x 23.4 x 2.8 cm

Buku ini adalah salah satu buku teks astronomi yang direkomendasikan untuk dibaca. Materi yang disampaikan tidak terlalu sulit dipahami oleh para pemula. Cakupan materi yang dibahas juga cukup luas dan dilengkapi dengan informasi - informasi terbaru. Sebagai pelengkap, disertakan juga software Starry Night Backyard dan Deep Space Explorer, yang sudah diakui sebagai salah satu software simulasi langit terbaik. Hanya saja paket buku teks ini tergolong sangat mahal, yaitu sekitar \$115.95 untuk buku baru.

### Mars

Sebuah video tentang Mars. Sayang video ini sudah agak lama sehingga mungkin sudah ada data yang ketinggalan jaman tetapi video ini boleh memberikan sedikit gambaran tentang Mars dan pertanyaan mengenai kemungkinan adanya kehidupan di Mars. Selamat belajar.

### Introduction to Observational Astrophysics

Narrated by Alan Alda, this introduction to observational astrophysics gives us a brief overview of the field and illuminates some of the interesting questions being currently researched.

## Tuesday, September 9, 2008

### Basic Properties of Stars

File berikut membahas tentang properti dasar dari bintang dalam observasi visual dan teknik-teknik dalam menghitung besaran-besaran fisik sebuah bintang. Semoga dapat bermanfaat.

Basic Properties of Stars

Review and further questions:
1. Sebutkan salah satu rasi bintang yang bintang paling terangnya adalah bintang dengan indeks beta, bukan alpha.
2. Jelaskan mengapa bintang nampak berkelap-kelip. Jelaskan juga mengapa planet tidak nampak berkelap-kelip.
3. Apakah kelap-kelip bintang ini menguntungkan atau merugikan pengamatan astronomis?. Jelaskan mengapa.
4. Tunjukkan 1 parsec sama dengan 3,26 tahun cahaya.
5. Sebutkan rentang magnitudo bintang yang bisa diamati manusia dengan mata telanjang. Hitunglah berapa kali lebih terang bintang paling redup yang bisa diamati manusia dengan mata telanjang dengan planet Venus saat terangnya maksimum. Bandingkan juga dengan bulan purnama.
6. Agar bisa mengamati bintang dengan magnitudo 20, perkirakan diameter teleskop yang diperlukan. Abaikan pengaruh atmosfer.
7. Bintang dengan magnitudo semu 8,5 dan sudut paralaks 0,002 detik busur. Hitunglah jarak dan magnitudo mutlaknya.
9. Sebuah garis dalam spektrum bintang X tampak pada panjang gelombang 5000 Angstrom. Hitunglah kecepatan radialnya jika garis tersebut nampak pada panjang gelombang 4998,2 Angstrom di laboratorium.
10. Hitunglah luminositas bintang dengan diameter 2 juta km dan temperatur permukaan 10.000 K. Jelaskan juga apakah bintang ini bisa nampak dengan mata telanjang jika sudut paralaks bintang ini 0,001 detik busur.
Jawaban Anda bisa disampaikan di bagian komentar. Jika ada yang sulit juga bisa ditanyakan lewat bagian komentar dan akan diusahakan di bantu. Selamat mencoba.

## Monday, September 8, 2008

### Simulation of Crab Supernova Explosion

Sumber : ESA/Hubble

PART 1

PART 2

## Sunday, September 7, 2008

### Daftar Materi (Silabus) Astrofisika 1

Sebagai penutup dari rangkaian soal latihan bab per bab dengan materi astrofisika dasar, di-upload-kan daftar materi lengkap untuk materi astrofisika dasar (astrofisika 1). Filenya dapat didownload dari link yang ada di bawah ini. (Sumber : Dr. Djoni N. Dawanas)

Daftar Materi Astrofisika 1

Untuk soal-soal dari bab-bab yang terkait astrofisika 1:

### Astrofisika : Bab 7 Bintang Ganda

Bab ini meliputi pembahasan tentang :
1. Bintang Ganda Visual
• Penentuan Massa Komponen Bintang Ganda Visual
• Hubungan Massa Luminositas
2. Bintang Ganda Astrometri
3. Bintang Ganda Spektroskopi
• Penentuan Massa Komponen Bintang Ganda Spektroskopi
4. Bintang Ganda Gerhana
• Penentuan Radius Komponen Bintang Ganda Spektroskopi
• Penentuan Massa Komponen Bintang Ganda Gerhana
Selamat belajar. (sumber soal : Dr. Djoni N. Dawanas)

Latihan Astrofisika Bab VII

(Untuk soal-soal dari bab-bab sebelumnya)

### Astrofisika : Bab 6 Gerak Bintang

Bab ini meliputi pembahasan tentang :
1. Gerak Sejati (Proper Motion)
2. Gerak Matahari
3. Paralaks Rata-Rata dan Paralaks Gugus
Selamat belajar. (sumber soal : Dr. Djoni N. Dawanas)

Latihan Astrofisika Bab VI

(Untuk soal-soal dari bab-bab sebelumnya)

## Thursday, September 4, 2008

### Trivia Quiz

Di bawah ini ada 2 buah gambar. Kedua gambar tersebut menunjukkan suatu fenomena astronomi tertentu. Coba tebak peristiwa apakah yang dimaksud? Dan kalau bisa, jelaskan juga apa penyebab timbulnya fenomena tersebut.

1.

2.
Jika Anda tahu, silakan isi comment dalam post ini. Jawaban akan di post minggu depan. Selamat mencoba.

## Wednesday, September 3, 2008

### The Sky at Einstein’s Feet

The insights of relativity have illuminated a century of astronomical discovery, often going beyond the phenomena that Einstein lived to see. This book shows, in nonmathematical ways, how deeply these ways of viewing the Universe have informed our interpretations of it, and how many of the amazing discoveries of these decades have made sense only as part of Einstein’s universe.

The author brings together the ways in which we see the bizarre effects of relativity played out on a cosmic scale. None of this is particularly new to practicing astronomers, but much has yet to be seen outside technical journals. The presentation avoids mathematics (except for the most famous equation in all of physics!), and is designed to be accessible to the interested public. Gravitational lenses, the visible effects of light-travel delays, the search for black holes, the ways relativity in atomic nuclei makes stars shine, are all treated. In many cases, some of the principals are still alive and provided new commentary on the discoveries. Numerous illustrations are newly produced from data in the archives of such observatories as Hubble and Chandra.

The Sky at Einstein’s Feet

### Our Star : The Sun

Our solar system is composed of the Sun and all things which orbit around it: the Earth, the other eight planets, asteroids, and comets. The Sun is 150 million kilometers (93 million miles) away from the Earth (this distance varies slightly throughout the year, because the Earth's orbit is an ellipse and not a perfect circle).

The Sun is an average star - there are other stars which are much hotter or much cooler, and intrinsically much brighter or fainter. However, since it is by far the closest star to the Earth, it looks bigger and brighter in our sky than any other star. With a diameter of about 1.4 million kilometers (860,000 miles) it would take 110 Earths strung together to be as long as the diameter of the Sun. The Sun is mostly made up of hydrogen (about 92.1% of the number of atoms, 75% of the mass). Helium can also be found in the Sun (7.8% of the number of atoms and 25% of the mass). The other 0.1% is made up of heavier elements, mainly carbon, nitrogen, oxygen, neon, magnesium, silicon and iron. The Sun is neither a solid nor a gas but is actually plasma. This plasma is tenuous and gaseous near the surface, but gets denser down towards the Sun's fusion core.

The Sun, as shown by the illustration to the left, can be divided into six layers. From the center out, the layers of the Sun are as follows: the solar interior composed of the core (which occupies the innermost quarter or so of the Sun's radius), the radiative zone, and the the convective zone, then there is the visible surface known as the photosphere, the chromosphere, and finally the outermost layer, the corona.
The energy produced through fusion in the Sun's core powers the Sun and produces all of the heat and light that we receive here on Earth. The process by which energy escapes from the Sun is very complex. Since we can't see inside the Sun, most of what astronomers know about this subject comes from combining theoretical models of the Sun's interior with observational facts such as the Sun's mass, surface temperature, and luminosity (total amount of energy output from the surface).

All of the energy that we detect as light and heat originates from nuclear reactions deep inside the Sun's high-temperature "core." This core extends about one quarter of the way from the center of Sun (where the temperature is around 15.7 million kelvin (K), or 28 million degrees Fahrenheit) to its surface, which is only 5778 K "cool".

Above this core, we can think of the Sun's interior as being like two nested spherical shells that surround the core. In the innermost shell, right above the core, energy is carried outwards by radiation. This "radiative zone" extends about three quarters of the way to the surface. The radiation does not travel directly outwards - in this part of the Sun's interior, the plasma density is very high, and the radiation gets bounced around countless numbers of times, following a zig-zag path outward.

It takes several houndred thousand years for radiation to make its way from the core to the top of the radiative zone! In the outermost of the two shells, where the temperature drops below 2,000,000 K (3.5 million degrees F) the plasma in the Sun's interior is too cool and opaque to allow radiation to pass. Instead, huge convection currents form and large bubbles of hot plasma move up towards the surface (similar to a boiling pot of water that is heated at the bottom by a stove). Compared to the amount of time it takes to get through the radiative zone, energy is transported very quickly through the outer convective zone.

The Sun's visible surface the photosphere is "only" about 5,800 K (10,000 degrees F). Just above the photosphere is a thin layer called the chromosphere. The name chromosphere is derived from the word chromos, the Greek word for color. It can be detected in red hydrogen-alpha light meaning that it appears bright red. Above the surface is a region of hot plasma called the corona. The corona is about 2 million K (3.6 million degrees F), much hotter than the visible surface, and it is even hotter in a flare. Why the atmosphere gets so hot has been a mystery for decades; SOHO's observations are helping to solve this mystery.

The Sun is not just a big bright ball. It has a complicated and changing magnetic field, which forms things like sunspots and active regions. The magnetic field sometimes changes explosively, spiting out clouds of plasma and energetic particles into space and sometimes even towards Earth. The solar magnetic field changes on an 11 year cycle. Every solar cycle, the number of sunspots, flares, and solar storms increases to a peak, which is known as the solar maximum. Then, after a few years of high activity, the Sun will ramp down to a few years of low activity, known as the solar minimum. This pattern is called the "sunspot cycle", the "solar cycle", or the "activity cycle".
Stars like the Sun shine for nine to ten billion years. The Sun is about 4.5 billion years old, judging by the age of moon rocks. Based on this information, current astrophysical theory predicts that the Sun will become a red giant in about five billion (5,000,000,000) years.

Source : SOHO Project

### International Year of Astronomy 2009

The International Astronomical Union (IAU) launched 2009 as the International Year of Astronomy (IYA2009) under the theme, The Universe, Yours to Discover. IYA2009 marks the 400th anniversary of the first astronomical observation through a telescope by Galileo Galilei. It will be a global celebration of astronomy and its contributions to society and culture, with a strong emphasis on education, public engagement and the involvement of young people, with events at national, regional and global levels throughout the whole of 2009. UNESCO has endorsed the IYA2009 and the United Nations proclaimed the year 2009 as the International Year of Astronomy on 20 December 2007.

The vision of the International Year of Astronomy (IYA2009) is to help the citizens of the world rediscover their place in the Universe through the day- and night time sky, and thereby engage a personal sense of wonder and discovery. All humans should realize the impact of astronomy and basic sciences on our daily lives, and understand better how scientific knowledge can contribute to a more equitable and peaceful society.

Astronomy is one of the oldest fundamental sciences. It continues to make a profound impact on our culture and is a powerful expression of the human intellect. Huge progress has been made in the last few decades. One hundred years ago we barely knew of the existence of our own Milky Way. Today we know that many billions of galaxies make up our Universe and that it originated approximately 13.7 billion years ago. One hundred years ago we had no means of knowing whether there were other solar systems in the Universe. Today we know of more than 200 planets around other stars in our galaxy and we are moving towards an understanding of how life might have first appeared. One hundred years ago we studied the sky using only optical telescopes and photographic plates. Today we observe the Universe from Earth and from space, from radio waves to gamma rays, using cutting edge technology. Media and public interest in astronomy have never been higher and major discoveries are frontpage news throughout the world. The IYA2009 will meet public demand for both information and involvement.

There are outstanding opportunities for everyone to participate in the IAU IYA2009 events. This brochure outlines some of the events planned at the global level, which will be supported by thousands of additional national and regional activities.

The IAU, UNESCO and our Organisational Associates wish everyone a year rich in astronomical experiences as we all celebrate the International Year of Astronomy 2009!

For resources in the form of powerpoint and PDF, click on the following links.
1. Powerpoint slides
2. PDF Version (Not the original version, this is more compressed versions)

This video below is the official trailer for the IYA 2009

Source : www.astronomy2009.org

## Monday, September 1, 2008

### Astrofisika : Bab 3 Besaran Mendasar dalam Astrofisika

Bab ini meliputi pembahasan tentang :
1. Besaran Matahari : Jarak, Massa, Luminositas, Radius, Temperatur Efektif Matahri
2. Jarak bintang : metode paralaks
3. Radius bintang : metode interferometri, okultasi Bulan, dan bintang ganda gerhana